Patients with SAD undergo major seasonal changes in eating behavior and generally overeat, crave carbohydrates and gain weight in the fall and winter. In order to explore this relationship, 16 patients with SAD and 16 age- and sex-matched controls were given two isocaloric lunches on two different days: a high-carbohydrate meal and a high-protein meal. Mood measurements (Profile of Mood States, POMS) taken before and after the meal indicated that both SAD patients and normals reported reduced levels of tension (P less than .01), depression (P less than .02) and anger (P less than .05) following the high-carbohydrate versus the high-protein meal. However, normals reported increased fatigue after a high-carbohydrate versus the high-protein meal. However, normals reported increased fatigue after a high-carbohydrate meal whereas patients showed decreased fatigue levels (P less than .05). These data suggest that carbohydrate craving in SAD may be part of a complex behavioral-biochemical feedback loop involving homeostatic regulation of brain chemistry, which presumably varies seasonally. The seasonal changes in eating and fat deposition in SAD patients may be part of an overall seasonal rhythm in energy utilization. Such rhythms occur in other animals, which may serve as useful models for further studies of SAD.